The exposed geology of the Capitol Reef area presents a record of mostly Mesozoic-aged sedimentation in an area of North America in and around Capitol Reef National Park, on the Colorado Plateau in southeastern Utah. Nearly 10,000 feet (3,000 m) of sedimentary strata are found in the Capitol Reef area, representing nearly 200 million years of geologic history of the south-central part of the U.S. state of Utah. These rocks range in age from Permian (as old as 270 million years old) to Cretaceous (as young as 80 million years old.) Rock layers in the area reveal ancient climates as varied as rivers and swamps (Chinle Formation), Sahara-like deserts (Navajo Sandstone), and shallow ocean (Mancos Shale). The area's first known sediments were laid down as a shallow sea invaded the land in the Permian. At first sandstone was deposited but limestone followed as the sea deepened. After the sea retreated in the Triassic, streams deposited silt before the area was uplifted and underwent erosion. Conglomerate followed by logs, sand, mud and wind-transported volcanic ash were later added. Mid to Late Triassic time saw increasing aridity, during which vast amounts of sandstone were laid down along with some deposits from slow-moving streams. As another sea started to return, it periodically flooded the area and left evaporite deposits. Barrier islands, sand bars and later, tidal flats, contributed sand for sandstone, followed by cobbles for conglomerate, and mud for shale. The sea retreated, leaving streams, lakes and swampy plains to become the resting place for sediments. Another sea, the Western Interior Seaway, returned in the Cretaceous and left more sandstone and shale only to disappear in the early Cenozoic. From 70 to 50 million years ago the Laramide orogeny, a major mountain building event in western North America, created the Rocky Mountains to the east. The uplift possibly acted on a buried fault to form the area's Waterpocket Fold. More recent uplift of the entire Colorado Plateau and the resulting erosion has exposed this fold at the surface only within the last 15 to 20 million years. Ice ages in the Pleistocene increased the rate of precipitation and erosion. The cracked upper parts of the Waterpocket Fold were especially affected and the fold itself was exposed and dissected.
Primary deposition of sediments Some important concepts: A formation is a formally named and defined geologic unit with unique characteristics. Those characteristics were created during a largely unbroken period of time and result from the specific depositional environment that the formation was laid down in. A member is a minor unit in a formation and a bed is a distinct subunit of a member. Groups are sets of formations that are related in significant ways such as, for example, all being deposited during a dry period that lasted millions of years or as the result of an ocean periodically flooding the same area over millions of years. The various kinds of unconformities are gaps in the geologic record. Such gaps can be due to a prolonged absence of deposition or due to subsequent erosion that removes previously deposited rock units. The following sections are ordered from oldest to youngest rock units in order to create a geologic history of events. This is the opposite order one would see in an actual cross section of the sediments because newer rock units are deposited on top of older ones per the law of superposition.
Cutler and Kaibab formations (Permian)
In early Permian time, Utah was on a continental shelf that was occasionally covered by a shallow arm of the Panthalassa Ocean. That part of Laurasia was on a passive continental margin not unlike the current west coast of equatorial Africa. The resultant formations are part of the approximately 290- to 250-million-year-old Cutler Formation (called a group locally) Utah was nearly on the paleoequator while the first members of the Cutler Formation were deposited but it had migrated nearly to 10° north latitude by around 275 million years ago. The Cutler records sedimentation during this time and is composed of four members (youngest to oldest):
White Rim Sandstone, (resistant caprock) Organ Rock Shale (locally missing), Cedar Mesa Sandstone, and the Elephant Canyon (locally buried). Only the two sandstone members of the Cutler Formation, the Cedar Mesa and White Rim, are exposed in the park but they cannot be easily distinguished from each other and are thus often treated as a single stratigraphic unit there. The White Rim and Cedar Mesa are composed of fossilized cross-bedded sand dunes that were likely deposited in an arid coastal environment that periodically flooded with sea water. Sand in these formations are somewhat sorted by size, well-rounded (worn by abrasion), and range from very fine- to medium-grained. Good outcrops of the locally 800 foot (240 m) thick Cedar Mesa and 420 foot (128 m) thick White Rim can be found in the bottom of Sulphur Creek and at the bottom of the Circle Cliffs outside the park's western border. In other areas the Organ Rock Shale is between the Cedar Mesa and White Rim but it pinches out east of the park. Both the locally buried Elephant Canyon and missing Organ Rock are exposed in nearby Canyonlands National Park 60 miles (100 km) east (see geology of the Canyonlands area). Later in Permian time, the Kaibab Sea invaded the land and laid down a limey ooze that later lithified to form the locally up to 200 foot (60 m) thick Kaibab Limestone. This is the same light gray to white formation that rims the Grand Canyon to the southwest (see Geology of the Grand Canyon area). The lower parts of the Kaibab were interbedded with sand and silt before its main component, limestone, was converted into chert-rich dolomite by the intrusion of magnesium. The formation contains fossils of invertebrates including brachiopods, bryozoans, crinoids, gastropods, and pelecypods. Outcrops of the cliff-forming Kaibab in Capitol Reef can only be seen in the deeper canyons located in the westernmost part of the park. Retreat of the Kaibab Sea by the Mid Permian exposed its seabed to erosion, resulting in 100 foot (30 m) deep channels and the creation of a gap in the geologic record called an unconformity.
Moenkopi Formation (Triassic)
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